We first write the global error as a sum of all local errors [1]}
\(|| A - A_{\text{H}}||_{\text{F}}^2 = \sum _{\tau \times \sigma \in P_{\text{admissible}}} || A\vert _{\tau \times \sigma }-A_{\text{H}}\vert _{\tau \times \sigma }||_F^2,\) 
